The present invention relates to a thread chain sewing method and device for use in the two-needle overlock sewing machine.
Generally speaking, in the case where over-edging is performed by the overlock sewing machine along the edge of a segment of fabric material, continuous thread chains are also formed, extending from the fabric material when the sewing process is finished.
Therefore, a thread chain holding and cutting means is provided in front of the needle drop points of the one-needle overlock sewing machine and serves to hold the free end of the thread chains extending from the fabric material, which are then cut off from the fabric material when the sewing process is finished.
Thread chains thus held by the thread chain holding and cutting means on the one-needle machine can be automatically folded and sewn into the seam of a subsequent segment of fabric material, thus preventing the seam at the sewing-start portion of the fabric material from being frayed, and without requiring the back-tacking operation of a back-tacking machine.
However, the two-needle overlock sewing machine had a drawback that prevented thread chains from being automatically smoothly folded and sewn into the seam. Namely, in the process of automatically sewing previously formed thread chains into the seam, the newly formed thread chains, which are wrapped around the chaining-off fingers, are also folded and sewn into the seam.
The two-needle overlock sewing machine has two needles and two chaining-off fingers, each arranged at one side of the needle drop point of each of the needles, that is, the inner and outer chaining-off fingers are arranged parallel to and alongside each other at both sides of the needle drop point of the outer needle. Therefore, thread chains which are formed before the sewing operation of the sewing machine is applied to the fabric material are formed wrapped wide around the inner and outer chaining-off fingers. As the result, thread chains held by the thread chain holding and cutting means are sewn together with newly formed ones. Similarly, when being sewn into the seam, thread chains are sewn to the fabric material by the thread of the outer needle, thus preventing the sewing of thread chains into the seam, leaving the thread chains projecting past the edge of fabric material at the beginning of seams.
In order to eliminate these drawbacks which are often seen in the conventional two-needle overlock sewing machine, the inventors of the present invention have developed a thread chain sewing device for use in the two-needle overlock sewing machine wherein the inner chaining-off finger, which was conventionally fixed parallel to and alongside the outer chaining-off finger, is arranged to be retractable in a direction opposite to the fabric material feeding direction and is held in its retracted position during the non-sewing time, thus allowing thread chains to be formed wrapped only around the outer chaining-off finger before the sewing operation of the sewing machine reaches the fabric material and preventing these thread chains from being sewn to the fabric material by the thread of the outer needle during sewing, with the result that these thread chains can be smoothly folded and sewn into the seam.
However, according to test conducted using the device, it has become apparent that if the inner chaining-off finger is moved to its retracted position at the moment when the sewing operation is finished, that is, at the time when the trailing end of fabric material passes through the needle drop points, thread chains formed after sewing are not wrapped around the inner chaining-off finger so as to be symmetrical and beautiful, and cannot be therefore beautifully folded and sewn into the seam when the sewing operation is applied to a subsequent segment of fabric material.
In addition, in the case where assembly seaming such as over-edging is performed with the overlock sewing machine, it is necessary that the needle threads be tightly tensioned, different from the case of blind stitch hemming. In the case of the stitched network shown in FIG. 1, for example, the tension of each of the threads is set in such a way that each of the looper threads B and C is made 3.5-5 times longer than each of needle threads A and A'. Therefore, thread chains formed in this manner after the sewing operation are wide and have no satisfactory elasticity. Such chains are not narrow enough to be folded and sewn into the seam of a subsequent segment of fabric material, because needle threads A, A' and looper threads B, C, together forming the thread chains, are remarkably different in length.